MAGNETIC INSTABILITY OF KONDO INSULATORS

The magnetic excitation spectra of Kondo insulators, is examined in th e vicinity of a magnetic transition. The Kondo insulators are modeled by the Anderson Lattice Hamiltonian, which has a hybridization gap bet ween the bands in the quasi-particle dispersion relations. At precisel y half-filling, the system becomes an indirect gap semiconductor. The spectrum of magnetic excitations contains contributions from spin-flip electron-hole excitations which form a continuum above a threshold en ergy, and a contribution from collective excitations. The spectrum of collective excitations softens at the magnetic instability. For ferrom agnetic systems, the magnetic instability is accompanied by an exchang e splitting which if sufficiently large can drive the system from the insulating state to a metallic state. The theory is applied to UNiSn. (C) 1998 Published by Elsevier Science B.V. All rights reserved.